Read recent history from the end of the file

[echo8134]

Summary

Loading a limited history parsed the entire file before truncation. Read regular files backward in bounded chunks until enough valid entries are found. Preserve newest-first ordering and the forward scanner for unlimited loads and nonregular files.

Related issues

No linked issue.

Verification

Public Load tests, bounded byte-count regression and real history picker checks pass. A five-iteration benchmark loads 1,000 entries from a 100,000-record file. Runtime drops from 82.49 ms to 1.68 ms, and allocated memory drops from 85.19 MB to 0.77 MB. Errors in older records outside the requested tail are intentionally unseen; recent line-size limits remain unchanged.

Local checks passed on Go 1.27.1. The CI workflow uses Go 1.26.3.

Checklist

• go build ./... and go test ./... pass
• gofmt -l . and go vet ./... are clean
• Added regression tests for the changed behavior
• cheatmd --lint examples/ passes
• Docs/wiki changes are not needed for this fix